EP0035756B1 - Process and apparatus for decreasing heat and mass transfer in the immediate wall surroundings of fluidized bed reactors - Google Patents

Description

The invention relates to a method and a device for reducing the heat, material and pulse exchange in the immediate vicinity of the wall in fluidized bed reactors, in which a granular solid is flown by a fluid from below and is thereby set into an intensive, whirled movement (fluidized bed).

The reduction in heat, material and pulse exchange is e.g. B. in chemical reactors or combustion chambers is often desirable in order to reduce heat loss or to protect the walls at the same time from corrosive and erosive stress.

Due to the intensive movement of solids, fluidized beds have z. B. an extraordinarily good heat transfer capacity. The heat transfer coefficients on the fluidized bed side are, for example, about an order of magnitude higher than in heat exchangers which are only exposed to gas (M. Baerns, Chem. Lng. Techn., 40 [1968], 737). This property, which is often quite desirable, can lead to undesirably high heat dissipation via the reactor walls in some applications or in certain operating phases. The mass transfer in fluidized beds is also greatly promoted by the swirling of the solid, so that mixing of solids in fluidized bed reactors can easily be carried out and lead to very homogeneous products.

In order to reduce undesirably high heat dissipation through the walls and / or corrosive and erosive attack, ceramic linings, e.g. B. by stamping or masonry, common (Lueger, Lexikon der Technik, 4th edition [1965], Vol. 7, page 266, Stuttgart). However, the peculiarity of the brick lining means that the number of wall penetrations is kept as low as possible at these points due to the increased susceptibility to failure of the lining. In fluidized bed reactors with high thermal stress on the reaction space, such as. B. high-performance combustion chambers, but must be cooled by a heat exchanger immersed in the layer in order to be able to regulate the reaction temperature. In this case, the walls are not suitable due to the large number of openings; one rather uses cooled fin tube walls, in which bushings are easy to manufacture and which have sufficient strength. The increased heat dissipation via the cooled walls must be accepted with this design (E. Wied, steam generator with fluidized bed combustion under atmospheric and excess pressure conditions, VGB-Kraftwerkstechnik, 58 [1978], 8, 554). In particular when heating the fluidized bed reactor, this has the effect that disproportionately large heating powers are required, which i. a. must be brought up by external energy.

Stamping out the reactor with a heat-insulating material has only a comparatively small effect, since the number of fastening pins in the wall only slightly reduces the heat transfer coefficient. In addition, the durability of the stamping out in the area of the many bushings, as are required for heat exchanger bundles, is very limited, and damage can only be eliminated with great effort. This also applies if a lining does not have to be installed for thermal insulation, but for protection against corrosive and erosive attack (W. Gumz, Short Manual of Fuel and Furnace Technology, 3rd edition, pages 600-603, Berlin, Göttingen, Heidelberg, 1962; Koppers Handbuch der Fueltechnik, 3rd edition, page 363, Essen).

The invention is therefore based on the object of overcoming the aforementioned disadvantages without significantly changing the favorable properties of the fluidized bed. In particular, the heat transfer through the reactor walls as well as the mass and momentum exchange near the wall should be significantly reduced.

It has been shown that this object can be achieved in a surprisingly simple and technically advanced manner according to the present invention if the flow resistance for the fluid near the wall is increased to such an extent that the fluidized bed is no longer whirled up in this area. This can be achieved by installing internals in the reaction apparatus which protrude from the reactor walls. According to the invention, these internals are preferably rib-shaped with a preferably horizontal extension. In order to particularly reliably prevent swirling near the wall, the spacing of the ribs in the flow direction of the fluid is at most about half as large as the rib height transversely thereto. The ribs can protrude perpendicularly from the wall or can be inclined. The internals can also be arranged according to the invention parallel to the reactor wall and form a gap therewith, the width u. a. depends on the desired insulation effect.

According to a further development of the invention, the internals according to the invention can be installed in segments, which reduces the thermal material stresses and simplifies the assembly and possible storage of such ribs and enables prefabrication and also subsequent installation.

Internals according to the invention can be attached to the inner wall of the reactor by welding, screwing, gluing or similar techniques will. In any case, they offer the advantage of a certain stiffening of the reactor wall and it is therefore possible to reduce the material dimensions and / or other stiffening, as are common especially in thermally stressed reactors. As far as the internals are those that are arranged parallel to the reactor wall and form a gap with it, it can be advantageous to use this z. B. give a scale structure to stabilize them against heat distortion. The latter is particularly recommended if the internals in question consist of sheet metal strips bent according to the shape of the reactor.

It has been shown that the insulation effect of the internals according to the invention is greater the wider the gaps or the further the ribs protrude into the reactor space.

By means of installations of this type, the flow resistance for the fluid in the immediate vicinity of the wall is increased to such an extent that the vortex movement of the solid, by which the good heat and mass transfer in fluidized beds is brought about, comes to a standstill in the peripheral zone. This result is surprising insofar as built-in components not close to the edge, e.g. B. heat exchanger tubes are surrounded by the fluidized bed in a liquid-like manner (Fluidization, J.F. Davidson, D. Harrison [eds.], London and New York [1971], eg Chapter 11).

It is of particular advantage in the described method that the fluidized material itself is used to protect the wall or to insulate the fluidized bed reactor. Therefore, no complex stamping or the like has to be carried out, but when the reactor is filled with layer material and the subsequent fluidization, the protective layer automatically forms in the rest zone. In addition, the material flows out of the rest zone when draining the reactor filling, for example when the apparatus is shut down, and gives the walls z. B. free for inspections.

If the flow resistance near the wall is increased by baffles, the baffles installed parallel to the wall are particularly suitable for draining the layer material completely out of the quiet zone, while rib-shaped baffles provide particularly good accessibility to the walls after the layer material has been drained and also e.g. B. can pass through between the rows of tubes of a heat exchanger bundle without difficulty. By tilting the ribs, etc., special effects can be achieved, e.g. B. complete drainage of the layer material when emptying the reactor or vice versa a retention of a particularly large amount of layer material on the walls even after emptying the reactor.

Another advantage is that there is no need to take into account the thermal shock resistance of brickwork or excavations, i. H. regardless of its protective equipment, the reactor can be heated or cooled as quickly as required.

Since the rest zone near the edge does not participate in the vortex movement, there is only a strongly inhibited mass transfer with the swirling core zone of the reactor. This is advantageous for reactions with corrosion-promoting reactants to protect the walls of the reactor, but also for reactions with strong heat, since not only is the heat transfer coefficient reduced in the quiet zone, but there is no or only a greatly reduced reaction. Furthermore, any erosive stress only occurs on the easily renewable internals, while the actual reactor wall is protected by the quiet zone.

With the aid of the drawing, an apparatus (enlarged section) for executing the method according to the invention is only explained in more detail, for example. The figure shows the arrangement and various forms of the internals which prevent the swirl movement in order to achieve a quiet zone in the vicinity of the wall of the reactor.

According to the figure - enlarged section of a conventional fluidized bed reactor - the latter is charged with solid (fluidized material) 1, which is whirled up from below with the fluid (gas or liquid) entering through the inflow base 2. The intensive movement of solids, which is the reason for the good mass and heat exchange within the layer and from the layer to the walls, is reduced, for example, by internals 6 to 11 in the immediate vicinity of the wall to such an extent that no swirling movement occurs and consequently mass and heat exchange be greatly reduced. If in the layer z. B. heat exchanger tubes 3 or transducers 4 or the like, which are guided through the reactor walls, the vortices inhibiting internals 6 to 10 z. B. advantageously be rib-shaped. As a result, the bushings can be laid in the spaces between the ribs, which leads to a particularly simple construction. The internals protruding from the fluidized bed reactor wall 5 can either be straight 6 or inclined 7, 8, angled upwards 9 or downwards 10 or can also be arranged parallel to the reactor wall 5. In the latter case, they can be held, for example, by spacers 12, which can be designed so that they additionally hinder the movement of solids. This creates a gap 13 with the inner wall of the reactor.

example 1

The effectiveness of the invention was demonstrated in a fluidized bed reactor model (length 400 mm, width 500 mm, height 800 mm) in the following manner:

Ribs (according to figure - reference numerals 6 to 10) of approximately 80 mm in height and approximately 40 mm from one another were selected as internals, which stood essentially perpendicularly from the inner wall of the reactor. Pipes were led through the wall between the internals, as shown in the figure; Eddy material was sand. The internals were only attached to one side of the rectangular model reactor, the walls of which were made of plexiglass. When the sand was whirled up, it became clear that intensive solid movement also occurred on the walls without fittings, also around the pipes passed through the wall, but in the area of the fittings according to the invention the sand was completely at rest.

Example 2

In a fluidized bed combustion chamber (length 400 mm, width 800 mm, fluid bed height 1000 mm) with a side wall cooled for test purposes (800 mm), various internals were installed in front of the cooled surface at a distance of about 50 mm parallel to it, which the Prevent turbulence and thus reduce heat dissipation through the cooled wall. It was found that in heating tests with carbon-containing fluidized material in this combustion chamber under otherwise completely identical conditions, it took only about half the time to reach the operating temperature of approximately 850 ° C. than when this combustion chamber was operated without the internals. From this result it can be concluded that the heat transfer to the cooled wall is reduced by more than about 30% by preventing the whirling up near the wall.

Claims (6)

1. A method of reducing the heat, impulse, and material exchange in the direct vicinity of the walls of fluidized bed reactors, characterized in that the flow resistance for the fluid passed through the fluidized bed is increased in the vicinity of the walls to such an extent that the fluidized bed no longer eddies in this region.

2. Apparatus for performing the method according to claim 1, characterized by mountings (6, ..., 12) projecting from the inner walls of the reactor.

3. Apparatus according to claim 2, characterized in that the mountings (6, ..., 10) are constructed in the form of fins.

4. Apparatus according to claim 3, characterized in that the distance between the fins in the direction of flow of the fluid is at most half as much as their height transversely thereto.

5. Apparatus according to claim 2, characterized in that the mountings (11) are disposed parallel to the reactor wall and form a gap (13) with it.

6. Apparatus according to one or more of claims 2 to 5, characterized in that the mountings (6, ..., 12) are disposed in the manner of segments.

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